What are methods human actors use to imitate robots?
The great acting teacher Stella Adler wrote about mannerisms being a powerful tool for actors. Method acting in general focuses on natural performances based roughly on understanding the mindset of the character portrayed.  

It's possible actors who have portrayed androids have observed industrial robots to inform their physicality, and many performances convey the idea, via movement, of a mechanical inner structure.  (It is often said that an "actor's body is their instrument".)

What is more interesting is actors trying to convey the cognitive structure of the androids.  

With Arnold, and Terminator robots in general, the baseline performance is decidedly robotic, to convey their inhumanity. But the more advanced Terminators are able to mimic naturalistic human mannerisms, and even established human characters, to trick humans. 

Lieutenant Data often used head motions, such as cocking his head slightly, to convey computation.  Here the character arc involved working to become more human, as this character draws heavily on Pinocchio, the wooden puppet that became a boy.  Overall Data's performance conveyed a lack of emotion, a definite reference to the logic-oriented Mr. Spock, although I recall episodes where Data experimented with "emotional circuits" and "humor circuits", where the output was intentionally inconsistent with natural human behavior.

Blade Runner, where the Tyrell Corporation's motto was "More Human than Human", presented the cutting edge Nexus-6 androids as having emotions, but, due to their artificially short life-spans, were portrayed as childlike in trying to reconcile extremely powerful feelings.  The Voight-Kampff Test, a form of Turing Test, used in the film to identify androids, relied on the emotional response to questions.  

The key plot point of Do Androids Dream of Electric Sheep, the novel the film was based on, utilized what would be formalized as evolutionary game theory to hypothesize that empathy is a natural function of intelligence sufficiently advanced.  Deckard, who may or may not have been an android, and Rachel, who definitely was, are both capable of love.  This capacity informed their performances, to the extent that the androids came off as more human than the actual humans, due to the depth of their emotion.  This is also reflected in Blade Runner 2049 via the girlfriend-bot Joi, who us the most limited android, but the most human character in the film per her capacity to love (or at least simulate it.)

In the recent HBO Westworld reboot, the Androids replicate natural human mannerisms when playing their designated roles, but reset to more mechanical mannerisms when acting under their own agency.  This is reflected in Ex Machina, where the android mimics human emotions to pass a Turing Test and trick the human subject, only to revert to purely alien mannerisms after the android is free.  ("Alien" here used in the sense of non-human--it's possible the android is sentient as it seems to convey some degree of emotion in regarding the simulated human skin it will wear.)

The most interesting recent android performance may come from the recent Alien: Covenant where Michael Fassbender plays two identical androids, David and Walter, which have two distinct neural structures.  (David has the capacity to be creative, where Walter cannot.  In the film it is mentioned that David made people uncomfortable, so the creative functions were removed from subsequent models.)  The key difference in the performance seems to be that David demonstrates passion, and even emotions, where Walter is more clearly "robotic".


In general, the underlying approach of actors seems to have been to show the androids being distinct from humans, drawing a clear, though sometimes subtle, contrast.   
Actors portraying androids have typically utilized robotic mannerisms to convey an artificial entity.